The major emphasis of the proposed research is to better understand the cellular aspects of immune responses in ectothermic animals. The broad objectives of the research are threefold. Firstly it is proposed to test the hypothesis that fish have two classes of lymphocytes comparable to T and B cells of mammals and birds. Testing this hypothesis will involve cell separation techniques employing monoclonal antibodies to catfish lymphocyte surface components combined with in vitro assays of lymphocyte functions. The second objective is to test the hypothesis that low temperature immunosupression in fish results primarly from a preferential effect on the generation of helper cells. Testing this hypothesis will involve in vivo and in vitro studies of temperature effects on the generation of antibody secreting cells to T-dependent an T-independent antigens. The third objective is to test the hypothesis that fish T cells have a different membrane composition than fish B cells and hence, without appropriate low temperature acclimation (homeoviscous adaptation), will not undergo mitogenic responses at lower temperatures as do fish B cells. Testing this hypothesis will involve studies on isolated catfish B and T cell plasma membranes and on cellular events occurring during mitogenic responses at various temperatures. Optimistically, these proposed studies will result in important information regarding the phylogenetic development of immune responses and the fundamental features of the activation, differentiation and interaction of lymphocytes.